1. Technical Field
The invention relates to machining systems utilizing electrical linear motors and, more particularly, to machining systems that use multiple tools, each independently positionable at high acceleration/deceleration rates.
2. Discussion of the Prior Art
Multi-axis positioners for light or heavy industrial machining use mechanical ball-screw drives (see, for example, U.S. Pat. Nos. 4,102,035 and 4,359,814). Such drives inherently suffer from slow wind-up which inhibits rapid positioning and thereby productivity. To increase productivity, a great number of machining cells need to be used, each at its own limited production rate. It is conventional in the U.S. automotive industry to shape a complex workpiece, such as an engine block or head, by transferring such workpiece, clamped on a fixture and pallet, along a series of machining stations where a specific surface is cut or finished by a dedicated tool (or cluster of dedicated tools) fed along a unitary axis. The workpiece must be transferred, with time-consuming effort, to other fixtures and/or pallets to expose a variety of faces to the feed axis of the tools. The percentage of in-cut time exercised by such a system is low due to the frequency of low speed workpiece transfer and due to the slow rates of tool positioning. Each tool carries out a task dedicated solely to one machining function with little modification over several years of use. The initial cost of fabricating and installing such nonflexible dedicated equipment with complex controls is very high not only due to their sophistication but also due to the large number of single purpose cells needed to complete the shaping of a specific engine block or head.
To spread out the initial high cost of equipment acquisition, minimum volume production requirements are imposed for such lines and such requirements are extremely high, i.e., 400,000-800,000 workpieces per year. Even if the capacity of a machining line, such as for a cylinder block, were reduced to 300,000 units per year (or 1000 units per day using two shifts) and dedicated multiple-spindle turret heads were incorporated in at least some of such machining cells (as is practiced by some Japanese automotive companies to introduce semiflexibility, see FIG. 1), the number of machining cells would still need to be at least about 40. This high number of machining cells is costly in initial aquisition, maintenance, and risk of down-time resulting from failure of a single cell. Such prior art manufacturing systems do not allow the automotive producer to respond quickly to market demand changes either for different engine block or head designs or for different volume levels of the existing block or head design.
It is an object of this invention to provide a new approach to machining systems that dramatically reduces the number of machining cells required in machining a given workpiece, permits continuous use of the machining line to flexibly produce different products in volumes such as 50,000-500,000 per year, improves repeatability and accuracy of machining, increases the number of machining tasks and rate of carrying out such tasks at each station, and reduces the handling time for workpieces between in-cut stages.